This is almost certainly Steatoda nobilis, one of six species of False Widow (Steatoda) now found in the UK. It is arguably the UK's most venomous spider, but they bite rarely and symptoms are usually no worse than a bee or wasp sting. Where there is one there are almost certainly more, but an awareness of this spider is all that is required. They are not dangerous per se - only a very few of the very few people bitten have suffered symptoms of medical importance.

This is topical at the moment having been in the news again this week - see further info here:

Many araneomorphae ('true spiders') are sexually dimorphic, meaning they are easy to tell apart. The most common difference is the size, and the swollen abdomen seen only in females.

Mature males are also indentifyable by 'hooks' or 'spurs' found on the 2nd segments of the forelegs, used for holding the female while mating (they mate face on, which is hazardous to say the least for the male). They also have clubs on their pedipalps (the two feelers on the 'head' (cephalothorax)) for holding sperm, which is inserted into the female's spermathecae.

Also, both sexes of spiders can bite! In some species, males are actually more likely to bite - the Sydney Funnel Web Atrax robustus - is a great example of this. There are very few dangerous spiders in the world though, so don't be scared of them! In the UK there are no aggressive or dangerous spiders. In other countries like Australia and in the US, it is wise to know which spiders are dangerous, but most are harmless.